Motor cortical reflex myoclonus: a case study with MEG

Cortical reflex myoclonus usually depends for its generation on the hyperexcitability of sensory cortex, which manifests itself as an enhanced somatosensory evoked potential (SEP). A 25-year-old female, presenting with involuntary jerky dorsiflexion of the left foot, was found to have cortical reflex myoclonus which was aggravated during intended movements. The jerks were also elicited by electrical stimulation of the posterior tibial nerve, although the SEP evoked by the same stimulus was normal in latency and amplitude. Both the spontaneous spikes and the premyoclonus spike demonstrated by jerk-locked back averaging were localized to the superior frontal gyrus, just anterior to the paracentral sulcus. Paired-pulse magnetic stimulation disclosed lack of inhibition in the right hemisphere leg motor area, whereas the excitability of sensory cortex as studied by paired SEP testing was normal. This suggests that the myoclonus was caused mainly by enhanced excitability of the motor cortex and that this alone was enough for the production of long loop reflexes. We propose to designate this type of cortical myoclonus as motor cortical reflex myoclonus. It is generated in the motor cortex, but is at the same time stimulus-sensitive.